1. Field of the Invention
The present invention relates to a fairlead mechanism for guiding the vertical and horizontal sweeps of movement of a reciprocating rope that is connected to a working tool of an excavating machine such as a dragline or the like.
2. Description of Related Art
A dragline generally comprises a mobile chassis that supports a boom 300 to 400 feet long. The lower end of the boom is pivotally mounted on the chassis to permit the angle of boom elevation to be adjusted. A sheave is rotatably mounted adjacent the top end of the boom. A main lift rope such as a steel wire wound cable has one end connected to a hoist drum on the chassis and its other end reeved over and suspended from the sheave at the top end of the boom. A working tool such as a dragline bucket is connected to the suspended end of the main lift rope and is raised and lowered by actuation of the hoist drum by the dragline operator.
The dragline is pivotable about a vertical axis to provide the boom and the suspended dragline bucket with a working radius which can be 300 feet or more. A dragline bucket drag rope has one end connected to the dragline bucket and its other end connected to a drag rope drum located on the dragline chassis adjacent the foot of the boom. The drag rope drum is selectively energized by the dragline operator to pull the dragline bucket, when it is on the ground, toward the dragline chassis to load the bucket. The drag rope drum is also operative to release the drag rope for travel away from the chassis in order to permit the loaded bucket to be dumped or to permit an empty bucket to be moved to a desired working position remote from the chassis.
During operation the dragline bucket may be raised in order to spoil overburden high on a spoil pile, or to reclaim a spoil pile or the dragline bucket may be lowered down into a mine pit for loading. The lateral swinging movements and the vertical up and down movements of the dragline bucket cause the drag rope to have both a vertical and a horizontal angular sweep of travel relative to the drag rope drum. During the vertical and horizontal sweeping movements the drag rope also reciprocates coward and away from the chassis, and travel of the drag rope after it leaves the drag rope drum must be guided to prevent undesirable contact with the dragline chassis. This guiding of the drag rope is accomplished by what is termed a fairlead mechanism. The drag rope passes through the fairlead mechanism after it leaves the drag rope drum.
The fairlead mechanism includes a pair of grooved horizontal sheaves rotatably mounted side by side on the chassis and a pair of grooved vertical upper and lower sheaves rotatably journaled on a subframe that is pivotally mounted on the chassis for oscillatory movement about a generally vertical axis. The planes in which the horizontal and vertical sheaves lie are arranged at right angles relative to each other. The drag rope upon leaving the drag rope drum first passes between the facing grooves of horizontal sheaves and then between the facing grooves of the vertical sheaves. The drag rope is a wire wound cable which may be up to 4.5 inches in diameter and have a length of 600 to 700 feet in order to permit the dragline bucket to be moved to its various working and dumping positions. The horizontal sheaves can weigh up to 11,200 lbs. each and the vertical sheaves up to 13,300 lbs. each. Usually a dragline will utilize two or more drag ropes and a corresponding number of fairlead mechanisms with all of the drag ropes being connected to a single dragline bucket.
Depending upon the direction of the drag rope sweep, the drag rope will have its primary frictional contact with either one or the other of the horizontal and vertical sheaves and cause the sheaves with which it has its primary frictional contact to rotate at a high rate of speed either clockwise or counterclockwise depending upon the direction of rope travel. When the angular directions of the rope sweeps are altered the primary frictional contact of the rope transfers from one sheave to the other and thus the fairlead sheaves are constantly being start-stop cycled by the changing frictional engagement with the drag rope. For example, assume that the drag rope is in contact with the lower vertical sheave which is being rotated by the drag rope. The upper noncontacted vertical sheave will be stationary. The drag rope is traveling at a high speed (up to 1000 feet per minute) and when the vertical sweep of the drag rope requires it to transfer its primary frictional contact to the upper stationary vertical sheave it will cause the upper sheave to start to rotate in the direction of travel of the drag rope. Due to the inertia of the heavy upper vertical sheave, initially there will be sliding frictional contact called scrubbing between the moving rope and the stationary upper vertical sheave until the rotational velocity of the upper vertical sheave matches the speed and direction of travel of the drag rope.
This scrubbing action creates rope wear and results in drag rope life of 1,000 to 1,500 hours. Draglines are normally operated 24 hours a day and the drag rope can require replacement every two months or less. Replacement of the drag rope is expensive and normally all drag ropes of the dragline are replaced at the same time in order to reduce costs. In order to replace the drag ropes a crawler tractor must be connected to pull each rope from its associated pull drum. Assuming the dragline utilizes two fairleads, two new drag ropes must be provided and connected to the drag rope drums. In addition, labor costs and lost production due to down-time are incurred.